Modern electromagnetic communication and remote sensing systems are using increasingly higher frequencies. High frequencies more readily accommodate the large bandwidths required by modern high data rate communications and by such sensing arrangements as chirp radar. Also, at higher frequencies the physical size of an antenna required to produce a given amount of gain is smaller than at lower frequencies. Some high frequencies are particularly advantageous or disadvantageous because of the physical transmission properties of the atmosphere at the particular frequency. For example, communications are disadvantageous at 23 gigahertz (GHz) because of the high path attenuation attributable to atmospheric water vapor, and at 55 GHz because of oxygen molecule absorption. On the other hand, frequencies near 40 GHz are particularly advantageous for communication and radar purposes in regions subject to smoke and dust because of the relatively low attenuation at those frequencies. When a high gain antenna array is required, it is advantageous for each antenna element in the array to have physically small dimensions in the arraying direction. For example, if it is desired to have a rectangular planar array of radiating elements for radiating in a direction normal or orthogonal to the plane of the array, it is desirable that the physical dimensions of each antenna element in the plane of the array be small so that they may be closely stacked. For those situations in which an antenna array uses a large number of radiating elements, it is also desirable that the radiating elements be substantially identical to each other so that the radiation patterns attributable to each radiating element are identical.
It is difficult to generate large amounts of radio frequency (RF) energy at microwave frequencies (frequencies roughly in range of 3 to 30 GHz) and at millimeter wave frequencies (roughly 30 to 300 GHz), and the losses attributable to transmission lines and other elements tend to be quite high. These problems tend to reduce the power available for radiation by an antenna. Good engineering design, such as minimization of transmission path lengths, can maximize the power available for radiation from an antenna. It may be desirable, however, to tune the antenna to maximize radiated power, select polarization, or to allow the antenna to operate efficiently at various frequencies within an operating frequency range.
Antennas in the form of a rectangular conductive patch separated by a layer of dielectric material from a ground plane are known to provide certain advantages for millimeter wave operation, such as reasonable impedance match. Such antennas also have a relatively broad beamwidth which is suitable for use in antenna arrays in which the beam scans a large angle. Furthermore, such antennas may be readily fabricated by photographic techniques and arrayed together with strip transmission lines formed on the dielectric substrate.
It is known to adjust the frequency and performance of such patch antennas, as described in U.S. Pat. No. 4,367,474 issued Jan. 4, 1983, in the name of Schaubert et al. The Schaubert arrangement describes the placing of conductive shorting posts in prepositioned holes extending between points on the patch antenna and an underlying ground plane. Schaubert also describes the replacing of the conductive shorting posts by switching diodes which are coupled to the ground plane by bypass capacitors and which are also coupled to an external electrical bias circuit by radio frequency chokes. At millimeter wave frequencies, the placement of the holes and of the connections of the diodes, and the necessary bias arrangements in the vicinity of the radiating portion of the antenna are subject to manufacturing tolerances which make it difficult to obtain reliable performance and which therefore increase the cost of manufacture of arrays which include multiple radiating elements. These problems are exacerbated by the radiation and stray coupling between the antennas and the electrical control lines coupled to the switching diodes for electrical bias thereof. It is desirable to increase the reliability of performance of tuned antenna elements for reduction of cost of manufacture and for ease of arraying.